Some vapour compression systems, such as refrigeration systems, heat pumps or air condition systems, are capable of operating in a subcritical control regime as well as in a supercritical control regime. Refrigerant flowing in the vapour compression system is compressed in a compressor and subsequently supplied to a heat rejecting heat exchanger. When the vapour compression system is operating in a subcritical control regime, the heat rejecting heat exchanger operates as a condenser, i.e. the compressed refrigerant is condensed while passing through the heat rejecting heat exchanger, and the refrigerant leaving the heat rejecting heat exchanger is therefore at least partly in a liquid state. On the other hand, when the vapour compression system is operating in a supercritical control regime, a phase transition of the refrigerant does not take place in the heat rejecting heat exchanger. Thus, in this case, the heat rejecting heat exchanger operates as a gas cooler, and the refrigerant leaving the heat rejecting heat exchanger is in a gaseous state.
Normally, one control strategy is used for the subcritical control regime, and a different control strategy is used for the supercritical control regime. This requires that the system is capable of keeping track of whether the vapour compression system operates in the subcritical or the supercritical regime. Furthermore, care must be taken when the vapour compression system is operated in the region close to the transitional point between the subcritical regime and the supercritical regime.
WO 2006/087005 A1 discloses a method for controlling an intermittently supercritically operating refrigeration circuit. In the subcritical mode a control valve is controlled so that a predetermined “subcritical pressure” ensuring a predetermined subcooling of the liquid refrigerant at the outlet of the heat rejecting heat exchanger is maintained. In the supercritical mode the control valve is controlled so that a predetermined “supercritical pressure”, which is optimized for optimum efficiency of the supercritical refrigerant at the outlet of the heat rejecting heat exchanger is maintained. In a border mode, in a region next to the critical point, the control valve is controlled dependent on a “continuity pressure” which is determined on the basis of the predetermined “subcritical pressure” and “supercritical pressure”. Thus, the refrigeration circuit is controlled according to three different control strategies.